Study of film thickness in elastohydrodynamic contacts by electrical capacitance

The current work, sponsored by the SKF Engineering and Research Centre in the Netherlands, is focused on studying aspects of lubrication relevant to rolling element bearings using electrical capacitance. This includes comparative film thickness measurements in glass-on-steel and steel-on-steel contacts, interaction of polar components with surfaces, as well as grease lubrication. It has been proven that the capacitive method can be successfully applied to very thin films, even down to around 10 nm thickness. The main part of the experimental work has been conducted on a test rig that simulates a contact between a ball and a raceway in a ball bearing. In this apparatus an EHD contact is formed between a steel ball and either a glass, or steel disc. For the first time it has been possible to perform parallel measurements of film thickness with an optical method, and electrical capacitance of an EHD contact, which allowed establishing an alternative approach to extracting quantitative film thickness values from the measured capacitance. This procedure has been further applied to a modified rolling element bearing in which all but one of the steel balls were replaced with non-conductive ceramic balls. By simplifying the experimental setup and focusing on a contact between a single ball and raceways it was possible to eliminate some of the system variables, such as unloaded region capacitance, thus giving a much clearer picture of the film thickness in a rolling bearing. The current study shows a high potential of the method giving further insight into the behaviour of lubricants in high-pressure contacts. The information obtained from measured capacitance can be treated as complementary to the output of other available techniques (optical interferometry, surface force apparatus, resistance, and ultrasound) providing a better understanding of the phenomena observed